Copolymer Of Ethylene And A Conjugated Diene, And Process For Its Production

ABSTRACT

A copolymer of ethylene and a conjugated diene is provided, in which ethylene content in the copolymer is greater than 20 mol %, the copolymer has a glass transition temperature between −110° C. and −90° C., and continuous methylene sequence lengths (MSL) in the copolymer are in a range of 12-162 methylene units. A method for producing the copolymer, comprising copolymerizing ethylene and a conjugated diene in the presence of a catalyst system to obtain the copolymer is also provided, in which the catalyst system comprises a heterocyclic-fused cyclopentadienyl rare-earth metal complex, an organoboron salt compound and an organoaluminum compound, wherein the heterocyclic-fused cyclopentadienyl rare-earth metal complex is represented by the structural formula:

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority benefits from Chinese patentapplication No. 201710234365.4, filed on Apr. 11, 2017, which is herebyincorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of rubber and, in particular,to a copolymer of ethylene and a conjugated diene, a process forproducing the copolymer, and a pneumatic tire comprising the copolymer.

BACKGROUND OF THE INVENTION

As a readily available monomer, ethylene has widespread use in theplastics industry. Historically, the price of butadiene, which is aby-product in the production of ethylene in petroleum processing, wascomparable to that of ethylene. Recently, the process of ethyleneproduction has changed, resulting in a decrease in the output of, and amarked increase in the price of, butadiene. In contrast, the price ofethylene has decreased. Therefore, using ethylene as a raw material forrubber products, such as those used in pneumatic tires, is attractive.For example, replacing butadiene with 10 wt % of ethylene in productionof polybutadiene used in tires could lead to large savings in rawmaterial costs. However, since the polymerization of a conjugated diene(such as butadiene) and that of an α-olefin (such as ethylene) occurunder different mechanisms, their copolymerization is difficult.Therefore, it is challenging to copolymerize ethylene and a conjugateddiene by the same catalytic system.

SUMMARY OF THE INVENTION

A copolymer of ethylene and a conjugated diene has a content of ethylenestructural unit greater than 20 mol %, preferably between 20 and 50 mol%.

In some embodiments, the copolymer has a glass transition temperature inthe range of −110° C. to −90° C., preferably between −105° C. to −95° C.

In some embodiments of the copolymer, the continuous methylene sequencelengths are in a range of 12-162 methylene units, preferably in therange of 12-100 methylene units, more preferably in the range of 12-70methylene units, and even more preferably in the range of 12-50methylene units.

In some embodiments, the copolymer has an average molecular weightbetween 10,000 and 1,000,000, preferably between 100,000 and 1,000,000,and more preferably between 100,000 and 500,000.

In some embodiments, the molecular weight distribution of the copolymeris between 1 and 10, preferably between 1 and 5, and more preferablybetween 1 and 3.

In some embodiments, the content of the cis-1,4 structural unit in theconjugated diene structural units in the copolymer is greater than 80%,preferably greater than 90%.

In some embodiments, the content of the trans-1,4 structural unit in theconjugated diene structural units in the copolymer is less than 20%,preferably less than 10%, and more preferably less than 5%.

In some embodiments, the copolymer has a tensile strength greater than 2MPa, preferably greater than 3 MPa, and more preferably greater than 4MPa.

In some embodiments, the copolymer has a break elongation greater than300%, preferably greater than 500%, and more preferably greater than600%.

The copolymer can be used for producing a rubber composite for apneumatic tire. The rubber composite can comprise the copolymer. In someembodiments, the rubber composite can also include other diene-basedelastomers such as natural rubber, polybutadiene, and styrene butadienerubber. The rubber composite can also contain one or more additives forproducing the composite, including fillers such as silica and carbonblack, curatives such as sulfur and accelerators, processing oils, fattyacids, waxes, resins, and antidegradants. The rubber composite can beused in various parts of a tire, such as the tread, the sidewall, andother parts made of rubber.

A method for producing the copolymer comprises performing acopolymerization reaction of ethylene and a conjugated diene in thepresence of a catalyst system comprising a heterocyclic-fusedcyclopentadienyl rare-earth metal complex represented by the followingStructural Formula 1:

wherein M is selected from the group consisting of scandium (Sc),yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium(Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb),dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb)and lutetium (Lu);

R₁, R₂, R₃, R₄ and R₅ are the same or different from each other and eachindependently selected from the group consisting of a hydrogen atom, analkyl radical having 1 to 10 carbon atoms, an alkyl radical having 1 to10 carbon atoms containing acetal, a alkyl radical having 1 to 10 carbonatoms containing ketal, an alkyl having 1 to 20 carbon atoms containingether, an alkenyl radical having 2 to 20 carbon atoms, an alkenylradical having 2 to 20 carbon atoms containing acetal, a alkenyl radicalhaving 2 to 20 carbon atoms containing ketal, an alkenyl radical having2 to 20 carbon atoms containing ether, an arylalkyl radical having 6 to20 carbons, an arylalkyl radical having 6 to 20 carbon atoms containingacetal, a arylalkyl radical having 6 to 20 carbon atoms containingketal, an arylalkyl radical having 6 to 20 carbon atoms containingether, a silyl radical having 1 to 14 carbon atoms, an silyl radicalhaving 1 to 14 carbon atoms containing acetal, a silyl radical having 1to 14 carbon atoms containing ketal, an silyl radical having 1 to 14carbon atoms containing ether, or two or more groups in R₁-R₅ connectedto each other to form an aliphatic or aromatic ring (for example, R₁ andR₂ are connected to each other to form a ring, or R₂ and R₃ areconnected to each other to form a ring, or R₄ and R₅ are connected toeach other to form a ring);

E is O, S, or N—R wherein R is methyl, phenyl or substituted phenyl;

X₁ and X₂ are single anionic ligands connected to a rare earth metal, X₁and X₂ are be the same or different, X₁ and X₂ are each independentlyselected from the group consisting of a hydrogen atom, a linear orbranched aliphatic radical or alicyclic radical having 1 to 20 carbonatoms, phenyl, a phenyl substituted by a linear or branched alkylradical having 1 to 20 carbon atoms or a cyclic aliphatic radical or anaromatic radical having 6 to 20 carbon atoms, a linear or branchedalkoxy radical having 1 to 20 carbon atoms, a linear or branchedalkylamine radical having 1 to 20 carbon atoms, a linear or branchedarylamine radical having 6 to 20 carbon atoms, a linear or branchedsilyl radical having 1 to 20 carbon atoms, borohydroxy radical, allyl,derivates of allyl, and halogen;

L_(w) is a neutral Lewis base (for example one of tetrahydrofuran,ether, ethylene glycol dimethyl ether, or pyridine); and w is an integerfrom 0 to 3.

In some embodiments, the catalyst system comprises a heterocyclic-fusedcyclopentadienyl rare-earth metal complex represented by StructuralFormula 1, an organoaluminum compound, and an organoboron salt compound.

The heterocyclic-fused cyclopentadienyl rare-earth metal complexrepresented by Structural Formula 1 can be synthesized by the proceduresdisclosed in Chen et al., Organometallics 2015, 34, 455-461 orWO2015/051569, and it is preferably one of Structural Formulas 2-5below:

In some embodiments, the organoaluminum compound contains at least onecarbon-aluminum bond and is represented by Structural Formula 6 below:

wherein R⁶ is selected from the group consisting of alkyl (includingcycloalkyl), alkoxy, aryl, alkaryl, arylalkyl radicals and hydrogen: R⁷is selected from the group consisting of alkyl (including cycloalkyl),aryl, alkaryl, arylalkyl radicals and hydrogen and R⁸ is selected fromthe group consisting of alkyl (including cycloalkyl), aryl, alkaryl andarylalkyl radicals. Representative compounds corresponding to theabove-mentioned constituents are: diethylaluminum hydride,di-n-propylaluminum hydride, di-n-butylaluminum hydride,diisobutylaluminum hydride, diphenylaluminum hydride, di-p-tolylaluminumhydride, dibenzylaluminum hydride, phenylethylaluminum hydride,phenyl-n-propylaluminum hydride, p-tolylethylaluminum hydride,p-tolyl-n-propylaluminum hydride, p-tolylisopropylaluminum hydride,benzylethylaluminum hydride, benzyl-n-propylaluminum hydride, andbenzylisopropylaluminum hydride and other organoaluminum hydrides. Alsoincluded are ethylaluminum dihydride, butylaluminum dihydride,isobutylaluminum dihydride, octylaluminum dihydride, amylaluminumdihydride and other organoaluminum dihydrides. Also included arediethylaluminum ethoxide and dipropylaluminum ethoxide. Also includedare trimethylaluminum, triethylaluminum, tri-n-propylaluminum,triisopropylaluminum, tri-n-butylaluminum, triisobutylaluminum,triamylaluminum, trihexylaluminum, tricyclohexylaluminum,trioctylaluminum, triphenylaluminum, tri-p-tolylaluminum,tribenzylaluminum, ethyldiphenylaluminum, ethyl-di-p-tolylaluminum,ethyldibenzylaluminum, diethylphenylaluminum, diethyl-p-tolylaluminum,diethylbenzylaluminum and other triorganoaluminum compounds.

The someorganoboron salt compound can be an ionic compound consisting ofan organoboron anion with a cation.

Examples of the organoboron anion include tetraphenylborate ([BPh₄]⁻),tetrakis(monofluorophenyl)borate ([B(C₆F₅)₄]⁻),tetrakis(difluorophenyl)borate, tetrakis(trifluorophenyl)borate,tetrakis(tetrafluorophenyl)borate, tetrakis(pentafluorophenyl)borate,tetrakis(tetrafluoromethylphenyl)borate, tetra(tolyl)borate,tetra(xylyl)borate, (tripheyl, pentafluorophenyl)borate,[tris(pentafluorophenyl), phenyl]borate, and decahydro-7,8-dicarbaniumborate.

Examples of the cation include a carbonium cation, an oxonium cation, anammonium cation, a phosphonium cation, a cycloheptatrienyl cation, and aferrocenium cation containing a transition metal. Here, the carboniumcation includes trisubstituted carbonium cations such as atriphenylcarbonium cation ([Ph₃C]⁺) and a tri(substitutedphenyl)carbonium cation, and a more specific example of thetri(substituted phenyl)carbonium cation includes atri(methylphenyl)carbonium cation. Examples of the ammonium cationinclude: trialkylammonium cations such as a trimethylammonium cation, atriethylammonium cation ([NEt₃H]⁺), a tripropylammonium cation, and atributylammonium cation; N,N-dialkylanilinium cations such as aN,N-dimethylanilinium cation ([PhNMe₂H]⁺), a N,N-diethylaniliniumcation, and a N,N-2,4,6-pentamethylanilinium cation; and dialkylammoniumcations such as a diisopropylammonium cation and a dicyclohexylammoniumcation. Specific examples of the phosphonium cation includetriarylphosphonium cations such as a triphenylphosphonium cation, atri(methylphenyl)phosphonium cation, and atri(dimethylphenyl)phosphonium cation.

Examples of the organoboron salt compound include [Ph₃C][B(C₆F₅)₄],[PhNMe₂H][BPh₄], and [NEt₃H][BPh₄], [PhNMe₂H][B(C₆F₅)₄].

An organoboron compound having the same function as that of theorganoboron salt compound, such as B(C₆F₅)₃, can also be used.

In some embodiments, the proportions of the components in the catalystsystem for producing the copolymer are as follows: The molar ratio ofthe organoaluminum compound to rare-earth metal M is adjustable between2:1 and 300:1, more preferably 4:1 to 200:1, most preferably between 8:1and 100:1. The molar ratio of the organoboron salt compound to metal Mis adjustable between 1:10 and 10:1, more preferably 0.5:1 and 10:1.

A wide range of catalyst amounts can be used to initiate thepolymerization. Normally, a low concentration of the catalyst system ismore desirable, since it can minimize, or at least reduce the productionof ash. Polymerizations occurs when the catalyst amount of therare-earth metal M varies between 0.05 and 1.0 mmol of metal per 100 gof monomer. A preferred proportion is between 0.1 and 0.3 mmol of metalper 100 g of monomer.

The concentration of the total catalyst system employed depends uponvarious factors, such as purity of the polymerization system,polymerization rate desired, temperature and other factors.

The polymerization reaction can be carried out in a wide range oftemperatures. For example, in some embodiments the polymerizationtemperature is adjustable between a lower temperature, such as −60° C.,and a higher temperature, such as 150° C. In some embodiments, thepolymerization temperature is preferably between 10° C. and 150° C.,more preferably between 25° C. and 150° C., and most preferably between25° C. and 90° C.

In some embodiments, the reaction pressure is not specifically limited.For example, in some embodiments, the polymerization reaction can beconducted at one atmospheric pressure or it can also be carried out atless than 1 atm or greater than 1 atm. The pressure of ethylene ispreferably 1 to 10 atm.

In some embodiments, the polymerization is carried out in a hydrocarbonsolvent. Suitable solvents include aliphatic saturated hydrocarbons,aromatic hydrocarbons, aryl halides or cycloalkanes, preferably one ofhexane, cyclohexane, benzene, toluene, xylene, chlorobenzene,dichlorobenzene, or bromobenzene, or mixtures thereof.

In some embodiments, the monomers used are ethylene and a conjugateddiene monomer. In some embodiments, the conjugated diene is selectedfrom conjugated dienes containing 4-20 carbon atoms, preferably is1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene,2-ethyl-1,3-butadiene, 2-phenyl-1,3-butadiene, 1,3-hexadiene,4-methyl-1,3-pentadiene, 1,3-pentadiene, 3-methyl-1,3-pentadiene,2,4-dimethyl-1,3-pentadiene, and 3-ethyl-1,3-pentadiene.

During the polymerization reaction, a solution of the conjugated dienemonomer can be added to the polymerization reaction system of anethylene-saturated solvent solution comprising the rare earth catalyst,an organoaluminum compound, and an organoboron salt compound.Preferably, the solution of the conjugated diene is gradually added tothe solution containing the ethylene, rare earth catalyst,organoaluminum compound, and organoboron salt compound. More preferably,the conjugated diene solution is added at a constant speed during thewhole polymerization reaction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a Differential Scanning calorimetry (DSC) analysis of thecopolymer produced in Example 1, according to an embodiment of thepresent copolymer and the process for its production.

FIG. 2 shows a DSC analysis of the copolymer produced in Example 1 aftera successive nucleation/annealing treatment.

FIG. 3 shows a DSC analysis of copolymer produced in Example 4 after asuccessive nucleation/annealing treatment.

FIG. 4 shows a ¹H Nuclear Magnetic Resonance (NMR) diagram of copolymerof Example 2.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS Examples 1-3

The heterocyclic-fused rare-earth metal complex of Structural Formula 5can be used as a catalyst precursor for catalyzing the copolymerizationof ethylene and butadiene by the following procedure, to produce acopolymer of ethylene and butadiene.

First, 250 mL toluene was added to a 2 L nitrogen-purged stainlessreactor vessel, and 1.0 atm ethylene was charged thereinto undervigorous stirring, so that a saturated state thereof in the toluenesolution was achieved. The complex of Structural Formula 5 (86 mg, 146μmol), Al^(i)Bu₃ (1.5 mL, 1.46 mmol, 1.0 M toluene solution) andtriphenylcarbonium tetrakis(pentafluorophenyl)borate, [Ph₃C][B(C₆F₅)₄](144 mg, 146 μmol) were dissolved into 20 mL of toluene, to produce acatalyst solution. The catalyst solution was then taken out from glovebox and quickly added to the polymerization reaction system at 40° C.,to initiate polymerization. Meanwhile, a toluene solution of butadiene(130 g solution, 26.7 wt % butadiene) was added into the polymerizationsystem at a constant speed. The speed of introduction of the butadienesolution was controlled by a flowmeter. Ethylene was continuouslyintroduced during the polymerization process. After the addition of thebutadiene solution, 20 mL methanol hydrochloride solution was addedimmediately to end the reaction. Then a large amount of ethanol wasadded to isolate the copolymer, which was dried under vacuum at 40° C.,until the weight of the polymer did not change.

Copolymer from Example 1 was obtained using the above method and wascharacterized. The results are shown below in Table 1.

Measurement of Content of Ethylene

The content of the ethylene structural unit in the copolymer wascalculated according to the ¹H NMR diagram of the copolymer measured inC₆D₄Cl₂ at 120° C. as follows:f_(E)=(I_((1.21-1.79))−1.5*I_(5.20))/(I_((1.21-1.79))+2*I_((5.50-6.00))−0.5*I_(5.20))*100%.

Measurement of the Glass Temperature (T_(g)) of the Copolymer

The glass temperature of the copolymer was measured by differentialscanning calorimetry (DSC) in accordance with GB/T 29611-2013.

Measurement of Continuous Methylene Sequence Length (MSL) in theCopolymer

Following the procedures of M. Zhang, S. E. Wanke, “Quantitativedetermination of short-chain branching content and distribution incommercial polyethylene by thermally fractionated differential scanningcalorimetry”, Polymer Engineering & Science, 2003, 43, 1878-1888, thecopolymer sample was subjected to a successive self-nucleation andannealing thermal analysis (SNA), so that continuous methylene sequenceswith different lengths (MSL) were classified. A set of segments havingsimilar continuous methylene sequence lengths resulted in crystalshaving similar sizes, and would melt at similar temperatures (F. M.Mirabella, J. Polym Sci: Part B: Polym Phys., 2001, 39, 2800). Next,melting endothermic peaks of the copolymer were measured by differentialscanning calorimetry (DSC) in accordance with GB/T 29611-2013. Eachendothermic peak corresponded to a continuous methylene sequence havingthe corresponding length. According to Equation I below, the lengths ofcontinuous methylene sequences in the copolymer were calculated.

$\begin{matrix}{{MSL} = \frac{2}{e^{({\frac{142.2}{T} - 0.3451})} - 1}} & (I)\end{matrix}$

In Equation I, MSL represents the length of continuous methylenesequence, and T is the peak value of the melting endothermic peak in theDSC curve.

Measurement of the Weight Average Molecular Weight (M_(n)) and theMolecular Weight Distribution (W_(w)/M_(n))

The weight average molecular weight (M_(n)) and the molecular weightdistribution (M_(w)/M_(n)) were measured by gel permeationchromatography (GPC) at 150° C., in which polystyrene was used as thestandard and 1,2,4-C₆Cl₃H₃ was used as the mobile phase.

Measurement of the Cis-1,4 Structural Unit and the Trans-1,4 StructuralUnit in Conjugated Diene Structural Units in the Copolymer

The cis-1,4 structural unit and the trans-1,4 structural unit inconjugated diene structural units in the copolymer were measuredaccording to the ¹³C NMR diagram of the polymer.

Measurement of the Tensile Strength and the Break Elongation of theCopolymer

The tensile strength and the break elongation of the copolymer weremeasured by a multifunctional mechanical tester in accordance withGB/T528-1998.

Copolymers in Example 2 (25° C.) and Example 3 (30° C.) were obtained bykeeping the other polymerization conditions constant, and varying thepolymerization temperature. The results are shown in Table 1.

TABLE 1 Example No. 1 2 3 Temperature, ° C. 40 25 30 Ethylene^(a), mol %45 29 36 trans-1,4 bond content 17 12 15 Mn(10⁴)^(b) 12.8 32.2 21.5Mw/Mn 1.68 1.72 1.71 T_(g) ^(c), ° C. −94.1 −95.7 −94.3 MSL^(d) 44-8812-50 30-70 ^(a)calculated by ¹H NMR diagram of the polymer;^(b)measured by GPC in 1,2,4-C₆Cl₃H₃ at 150° C. using polystyrenestandard; ^(c)T_(g) was measured by DSC according to GB/T 29611-2013;^(d)SNA was done over a temperature range of 62 to 132° C. using aheating rate of 10° C./min, annealing time of 15 min and cooling rate of10° C./min with a 5° C. increment between successive heating operations.DSCs of SNA treated samples were done according to GB/T 29611-201. MSLswere calculated by equation (I).

As shown in Table 1, increasing the polymerization temperature increasedthe ethylene content and the trans-1,4 structural unit content inconjugated diene structural units in the obtained copolymer. Thisresulted in that lengths of the continuous methylene sequences in thecopolymer becoming longer. The glass transition temperature of thecopolymer also increased slightly.

Examples 4-8

The copolymers of Examples 4-8 were produced using a catalystrepresented by Structural Formula 3, following the general procedure ofExample 1, by changing the addition speed of the butadiene solution. Theresults are shown in Table 3. Compared to the copolymer afforded byusing the catalyst represented by Structural Formula 5, the copolymersproduced by using the Structural Formula 3 catalyst have a lower T_(g)between −101° C. and −103° C.; the contents of trans-1,4 structural unitin conjugated diene structural units decreased to between 7 and 9 mol %;and the range of the continuous methylene sequence lengths in thecopolymer widened to between 44 and 160 methylene groups. Increasing thespeed of the addition of butadiene solution decreased the content ofethylene structural unit in the resultant copolymer from 41.7 mol % to17 mol %. This is shown in Table 2 below.

TABLE 2^(A) Example No. 4 5 6 7 8 V_(BD) ^(b), mL · min⁻¹ 3 5 7 9 12Ethylene^(c), mol % 41.7 35.6 28.8 24.6 17 trans-1,4 bond content^(d) 89 8 7 8 Mn(10⁴)^(e) 8.2 13.3 12.2 13.1 14.9 Mw/Mn 1.82 1.67 1.80 1.601.85 T_(g) ^(f), ° C. −102 −101 −102 −103 −102 MSL^(g) 74-160 60-15456-150 50-142 44-138 ^(a)Polymerization conditions: toluene, 80 μmol ofSc, 80 μmol of [Ph₃C][B(C₆F₅)], 800 μmol of Al^(i)Bu₃, ethylene 1 atm, T= 40° C. ^(b)The addition speed of butadiene solution. ^(c)Determined by¹H NMR spectroscopy in C₆D₄Cl₂ at 125° C. ^(d)Determined by ¹³C NMRspectroscopy in C₆D₄Cl₂ at 125° C. ^(e)Measured by GPC in 1,2,4-C₆Cl₃H₃at 150° C. using polystyrene standard. ^(f)Tg was measured by DSC.^(g)MSL represents the length of methylene sequences calculated byequation (I).

The tensile strength and the break elongation of the copolymers weremeasured by a multifunctional mechanical tester in accordance withGB/T528-1998. The results are shown in Table 3 below.

TABLE 3 Example No. 3 5 7 Ethylene, mol % 36 35.6 24.6 MSL 30-70 60-15456-150 Tensile Strength, MPa 3.28 4.0 3.15 Elongation at Break, % 650360 630

As seen in Table 3, when the ethylene structural unit contents in thecopolymers are similar, the copolymer of Example 3 with shorter ethylenesequence lengths shows a significantly higher break elongation than thecopolymer of Example 5, which has longer ethylene sequence lengths. Thecopolymer of Example 7, which has a smaller ethylene structural unitcontent, has a break elongation similar to the copolymer of Example 3,even though it has longer ethylene sequences.

While particular elements, embodiments and applications of the presentinvention have been shown and described, it will be understood, that theinvention is not limited thereto since modifications can be made bythose skilled in the art without departing from the scope of the presentdisclosure, particularly in light of the foregoing teachings.

What is claimed is:
 1. A copolymer of ethylene and a conjugated diene,wherein ethylene content in the copolymer is larger than 20 mol %, thecopolymer has a glass transition temperature between −110° C. and −90°C., and continuous methylene sequence lengths (MSL) in the copolymer arein a range of 12-162 methylene units.
 2. The copolymer of claim 1,wherein the conjugated diene is selected from butadiene, isoprene,2,3-dimethyl-1,3-butadiene, 2-ethyl-1,3-butadiene, 1,3-hexadiene,2-phenyl-1,3-butadiene, 4-methyl-1,3-pentadiene, 1,3-pentadiene,3-methyl-1,3-pentadiene, 2,4-dimethyl-1,3-pentadiene, and3-ethyl-1,3-pentadiene.
 3. The copolymer of claim 1, wherein ethylenecontent in the copolymer is between 20 mol % and 50 mol %.
 4. Thecopolymer of claim 1, wherein the copolymer has a number averagemolecular weight ranging from 100,000 to 500,000.
 5. The copolymer ofclaim 1, wherein the copolymer has a molecular weight distribution lessthan
 5. 6. The copolymer of claim 1, wherein trans-1,4 structure contentin conjugated diene units is less than 20 mol %.
 7. The copolymer ofclaim 1, wherein the MSL ranges from 12 to
 100. 8. The copolymer ofclaim 1, wherein the MSL ranges from 12 to
 70. 9. The copolymer of claim1, wherein, the MSL ranges from 12 to
 50. 10. A pneumatic tirecomprising the copolymer of claim
 1. 11. A method for producing thecopolymer of claim 1, comprising copolymerizing ethylene and aconjugated diene in the presence of a catalyst system to obtain thecopolymer, wherein the catalyst system comprises a heterocyclic-fusedcyclopentadienyl rare-earth metal complex, an organoboron salt compoundand an organoaluminum compound, wherein the heterocyclic-fusedcyclopentadienyl rare-earth metal complex is represented by StructuralFormula (1):

wherein M is selected from the group consisting of scandium (Sc),yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium(Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb),dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium(Yb), and lutetium (Lu); wherein R¹, R², R³, R⁴ and R⁵ are the same ordifferent from each other and each is independently selected from thegroup consisting of a hydrogen atom, an alkyl radical having 1 to 10carbon atoms, a C1 to C10 alkyl radical containing acetal, a C1 to C10alkyl radical containing ketal, a C1 to C20 alkyl containing an ethergroup, a C2 to C20 alkenyl radical containing acetal, a C2 to C20alkenyl radical containing ketal, a C2 to C20 alkenyl radical containingan ether group, a C6 to C20 aryl radical, a C6 to C20 aryl radicalcontaining acetal, a C6 to C20 arylalkyl radical containing ketal, a C6to C20 arylalkyl radical containing an ether group, a C1 to C14 silylradical, a C1 to C14 silyl radical containing acetal, a C1 to C14 silylradical containing ketal, and a C1 to C14 silyl radical containing anether group, or R¹ and R² are connected to each other to form a ring, orR² and R³ are connected to each other to form a ring, or R⁴ and R⁵ areconnected to each other to form a ring; wherein E is O, S, or N—R,wherein R is methyl, a benzene ring or a substituted benzene ring;wherein X¹ and X² are each independently selected from the groupconsisting of hydrogen, a C1 to C20 aliphatic radical, a C1 to C20alicyclic radical, phenyl, a substituted phenyl, a C1 to C20 alkoxyradical, a C6 to C20 alkylamine radical, a C1 to C20 arylamine radical,a C1 to C20 silyl radical, allyl, derivates of allyl, borohydroxyradical, and halogen; wherein the substituted phenyl is a phenylsubstituted by one or more of a C1 to C20 aliphatic radical, a C1 to C20alicyclic radical or a C6 to C20 aromatic radical; and wherein L_(w) isa neutral Lewis base and w is an integer from 0 to
 3. 12. The method ofclaim 11, wherein L_(w) is selected from the group consisting oftetrahydrofuran, ether and ethylene glycol dimethyl ether.
 13. Themethod of claim 11, wherein the organoboron salt compound is selectedfrom the group consisting of [Ph₃C][B(C₆F₅)₄], [PhNMe₂H][BPh₄],[NEt₃H][BPh₄] and [PhNMe₂H][B(C₆F₅)₄].
 14. The method of claim 11,wherein the organoaluminum compound is selected from the groupconsisting of trimethyl aluminum, triethyl aluminum, tripropyl aluminum,tributyl aluminum, triisopropyl aluminum, triisobutyl aluminum, trimlyaluminum, trihexyl aluminum, tricyclohexyl aluminum, trioctyl aluminum,triphenyl aluminum, tri-p-tolyl aluminum, tribenzyl aluminum, ethyldibenzyl aluminum and ethyl di(p-tolyl) aluminum.
 15. The method ofclaim 11, wherein a mole ratio of the organoboron salt compound to theheterocyclic-fused cyclopentadienyl rare-earth metal complex representedby Structural Formula (1) is in the range of 0.5:1 to 10:1, and a ratioof the organoaluminum compound to the heterocyclic-fusedcyclopentadienyl rare-earth metal complex represented by StructuralFormula (1) is in a range of 2:1 to 300:1.
 16. The method of claim 11,wherein the polymerization is performed in a solvent selected from thegroup consisting of aliphatic saturated hydrocarbons, aromatichydrocarbons, aryl halides, cycloalkanes, and mixtures thereof.
 17. Themethod of claim 16, wherein the solvent is selected from the groupconsisting of hexane, cyclohexane, benzene, toluene, xylene,chlorobenzene, dichlorobenzene, bromobenzene, and mixtures thereof. 18.The method of claim 11, wherein the polymerization is performed at atemperature ranging from 25° C. to 150° C.
 19. The method of claim 11,wherein the polymerization is performed at an ethylene pressure rangingfrom 1 atm to 10 atm.